After being cut from the Dungeon Master's Guide the mass combat rules have made their way into a free online supplement.
The designers of Dungeons & Dragons' Fifth Edition have released a free supplement with a playtest version of their Battlesystem mass combat rules. The rules allow for a few hundred foes to go up against each other in an abstracted, grid-based battle using the standard statistics found in the Monster Manual or Player's Handbook. The designers caution that the rules as they're written right now are a playtest version, saying that they're "written in pencil, not ink."
The rules use the same d20-based combat core as the standard D&D game, focusing on "stands" of ten soldiers each organized into "units." Each stand uses the same statistics as a single combatant of that type, essentially abstracting combat out to a slightly larger degree. Some rudimentary rules account for positioning units, fighting in formation, and armies breaking or routing from low morale. Heroes and magic get their own rules, showing how various spells and spellcasters, as well as giant monsters, might perform on the battlefield.
It's clear the rules aren't for the kinds of epic clashes you see in properties like The Lord of the Rings, since the scale is rather small, but it'll suffice for the kind of skirmishes a border baron or count gets into with their neighbors - conflicts involving a few hundred participants on either side. For anything bigger, for now, you're out in the cold unless you'd like to spend a whole day slugging it out.
If you're interested in what has changed during development, go check out news from last year on the D&D mass combat system.